Playing efficiency is calculated by how a count performs for a static set of playing decisions. That is about as much as I have been able to find out when I was curious.

Generally the wider you spread the more importance BC becomes compared to PE. With fewer decks used or left to be played PE becomes more valuable. So with spreads being confined by heat concerns of pitch games and the fewer cards used PE becomes at least as important as BC if not more important. But in shoe games, especially with poor cuts, where large spreads are generally tolerated BC becomes much more important than PE. That all said what really matters is how playing interacts with your ramp not the BC and PE stats. A sim can tell you all you need to know.